Technical Field
The present disclosure relates to a carrier, a two-component developer, an image forming apparatus, a process cartridge, and an image forming method.
Description of the Related Art
In a typical electrophotographic image forming process, an electrostatic latent image is formed on an electrostatic latent image bearer and then developed into a toner image with a developer. The toner image is transferred onto a recording medium, fixed thereon, and output as a printed matter.
The developer generally includes a toner and a carrier. A typical carrier is composed of a core material and a resin layer that is coating the core material. Conventionally, the resin layer is formed of low-surface-energy resins, such as fluororesin and silicone resin, for the purpose of extending the lifespan of the carrier. Such a resin layer forms of a uniform surface of the carrier, which prevents the occurrence of toner filming, an oxidization of the surface, a decrease in humidity resistance, and an adhesion of the carrier to a surface of a photoconductor, while extending the lifespan of the developer, protecting the surface of the photoconductor from scratch and abrasion, controlling charge polarity, and adjusting the amount of charge.
In accordance with recent demands for higher printing speed, reduction of environmental burden of wastes, and reduction of printing cost per sheet, there is a need for a highly-durable carrier.
Among various properties of the carrier, a resistance value is adjusted depending on the image forming system used in combination with the carrier, for achieving a target printing quality. One method of adjusting the resistance value involves including a conductive particle in the resin layer of the carrier. Examples of the conductive particle generally include carbon black, titanium oxide, zinc oxidize, and ITO (indium tin oxide). In particular, single-particle carbon black and ITO-coated particle have been reported as the conductive particle having excellent conductivity.
With respect to ITO-coated particle, ITO is required to be in the form of a thin coating film (hereinafter “conductive coating film”) on the surface of the base particle, for adjusting conductivity. When such an ITO-coated particle is used as the conductive particle in a carrier, however, since each carrier particle collides with the other carrier particles in a developing device, the conductive coating films of the ITO-coated particles which are exposed at the surface of the carrier particle are scraped off. Since the conductive coating film is thin, the base particle is exposed at the surface of the carrier particle in the early stage. As a result, the resin layer of the carrier rapidly weakens in terms of impact resistance, thereby accelerating an abrasion of the resin layer, a reduction of the resistance, an occurrence of carrier scattering, and shortening the lifespan of the carrier.
In view of this situation, there has been an attempt to more thickening the conductive coating film to delay an exposure of the base particle. However, this attempt does not prevent an abrasion of the conductive coating film.
On the other hand, the resin layer of the carrier may further include a conductive filler for giving conductivity to the carrier. Examples of the conductive filler generally include carbon black, available at a low price. Carbon black as the conductive filler, however, cannot respond to recent demands for higher printing speed, improved stress resistance, and an extended lifespan, because of causing a color contamination problem when the carrier is used in combination with a colored toner (especially yellow toner), white toner, or transparent toner.
Other examples of the conductive filler include those having a core-shell structure, in which the core and the shell respectively serve as the base particle and the conductive coating film. Such a core-shell-type conductive filler secures temporal charging ability of the carrier as the base particle is gradually exposed after the conductive coating film has been abraded. However, the core-shell type conductive filler has a drawback that the temporal charging ability fluctuates depending on the degree of abrasion of the conductive coating film (or the degree of exposure of the base particle). There is a demand for a carrier that more reliably provides temporal charging ability.